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Chapter 1: Light — Reflection and Refraction

NCERT Solutions for CBSE Class 10 Physics — 311 solved questions with detailed explanations.

311
Questions
13
Topics

Important Formulas

New Cartesian Sign Convention

All measurements are taken from the pole (P) of the mirror along the principal axis:

Practical Implications

Mirror Formula

The relationship between object distance (u), image distance (v), and focal length (f) is:

1/v + 1/u = 1/f

This formula is valid for all spherical mirrors and all positions of the object. Always use the sign convention when substituting values.

Magnification

Magnification (m) tells us how much larger or smaller the image is compared to the object:

m = h'/h = -v/u

Where:

Interpreting Magnification

Laws of Refraction

First Law: The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.

Second Law (Snell's Law): The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media. This constant is called the refractive index.

n₁ sin i = n₂ sin r

Or equivalently: sin i / sin r = n₂ / n₁ = n₂₁

Where n₂₁ is the refractive index of medium 2 with respect to medium 1.

Refractive Index

The absolute refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium:

n = c / v

Where c = 3 × 108 m/s (speed in vacuum) and v = speed in the medium.

Common Refractive Indices

A higher refractive index means the medium is optically denser — light travels more slowly through it.

Lens Formula

1/v - 1/u = 1/f

Where u = object distance, v = image distance, f = focal length. Note the minus sign — this is different from the mirror formula!

Magnification for Lenses

m = h'/h = v/u

Note: Unlike mirrors, there is no negative sign in front of v/u for lenses.

Power of a Lens

The power of a lens is a measure of how strongly it converges or diverges light.

P = 1/f

Where f is in metres and P is in dioptres (D).

Example: A convex lens of focal length 20 cm = 0.2 m has power P = 1/0.2 = +5 D.

Combination of Lenses

When multiple thin lenses are placed in contact, the net power is the algebraic sum:

P = P₁ + P₂ + P₃ + ...

This is how opticians prescribe spectacles — they combine lens powers to achieve the exact correction needed.

Solved Questions

Q1. The angle of incidence is equal to the angle of reflection. This is the statement of which law?

Difficulty: Easy · Topic: Laws of Reflection

Solution

The first law of reflection states that the angle of incidence (∠i) is always equal to the angle of reflection (∠r). The second law states that the incident ray, reflected ray, and normal all lie in the same plane. Snell's law and the law of refraction deal with refraction, not reflection.

Q2. A spherical mirror has radius of curvature 46 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{46}{2} = 23\) cm

Q3. A spherical mirror has radius of curvature 55 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{55}{2} = 27\) cm

Q4. A spherical mirror has radius of curvature 7 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{7}{2} = 3\) cm

Q5. A spherical mirror has radius of curvature 22 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{22}{2} = 11\) cm

Q6. A spherical mirror has radius of curvature 59 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{59}{2} = 29\) cm

Q7. A spherical mirror has radius of curvature 35 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{35}{2} = 17\) cm

Q8. A spherical mirror has radius of curvature 50 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{50}{2} = 25\) cm

Q9. A spherical mirror has radius of curvature 17 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{17}{2} = 8\) cm

Q10. A spherical mirror has radius of curvature 28 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{28}{2} = 14\) cm

Q11. A spherical mirror has radius of curvature 8 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{8}{2} = 4\) cm

Q12. A spherical mirror has radius of curvature 38 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{38}{2} = 19\) cm

Q13. A spherical mirror has radius of curvature 6 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{6}{2} = 3\) cm

Q14. A spherical mirror has radius of curvature 48 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{48}{2} = 24\) cm

Q15. A spherical mirror has radius of curvature 19 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{19}{2} = 9\) cm

Q16. A spherical mirror has radius of curvature 34 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{34}{2} = 17\) cm

Q17. A spherical mirror has radius of curvature 43 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{43}{2} = 21\) cm

Q18. A spherical mirror has radius of curvature 54 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{54}{2} = 27\) cm

Q19. A spherical mirror has radius of curvature 57 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{57}{2} = 28\) cm

Q20. A spherical mirror has radius of curvature 24 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{24}{2} = 12\) cm

Q21. A spherical mirror has radius of curvature 10 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{10}{2} = 5\) cm

Q22. A spherical mirror has radius of curvature 53 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{53}{2} = 26\) cm

Q23. A spherical mirror has radius of curvature 21 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{21}{2} = 10\) cm

Q24. A spherical mirror has radius of curvature 9 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{9}{2} = 4\) cm

Q25. A spherical mirror has radius of curvature 15 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{15}{2} = 7\) cm

Q26. A convex lens has focal length 40 cm. Find its power in dioptres.

Difficulty: Easy · Topic: Power of a lens

Solution

Power \(= \frac{1}{f(\text{in m})} = \frac{100}{40} = 2.5\) D

Q27. A convex lens has focal length 30 cm. Find its power in dioptres.

Difficulty: Easy · Topic: Power of a lens

Solution

Power \(= \frac{1}{f(\text{in m})} = \frac{100}{30} = 3.33\) D

Q28. A convex lens has focal length 20 cm. Find its power in dioptres.

Difficulty: Easy · Topic: Power of a lens

Solution

Power \(= \frac{1}{f(\text{in m})} = \frac{100}{20} = 5.0\) D

Q29. A convex lens has focal length 25 cm. Find its power in dioptres.

Difficulty: Easy · Topic: Power of a lens

Solution

Power \(= \frac{1}{f(\text{in m})} = \frac{100}{25} = 4.0\) D

Q30. A convex lens has focal length 15 cm. Find its power in dioptres.

Difficulty: Easy · Topic: Power of a lens

Solution

Power \(= \frac{1}{f(\text{in m})} = \frac{100}{15} = 6.67\) D

Q31. A convex lens has focal length 10 cm. Find its power in dioptres.

Difficulty: Easy · Topic: Power of a lens

Solution

Power \(= \frac{1}{f(\text{in m})} = \frac{100}{10} = 10.0\) D

Q32. A convex lens has focal length 50 cm. Find its power in dioptres.

Difficulty: Easy · Topic: Power of a lens

Solution

Power \(= \frac{1}{f(\text{in m})} = \frac{100}{50} = 2.0\) D

Q33. A spherical mirror has radius of curvature 60 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{60}{2} = 30\) cm

Q34. A spherical mirror has radius of curvature 29 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{29}{2} = 14\) cm

Q35. A spherical mirror has radius of curvature 14 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{14}{2} = 7\) cm

Q36. A spherical mirror has radius of curvature 20 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{20}{2} = 10\) cm

Q37. A spherical mirror has radius of curvature 58 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{58}{2} = 29\) cm

Q38. A spherical mirror has radius of curvature 52 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{52}{2} = 26\) cm

Q39. A spherical mirror has radius of curvature 11 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{11}{2} = 5\) cm

Q40. A spherical mirror has radius of curvature 23 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{23}{2} = 11\) cm

Q41. A spherical mirror has radius of curvature 31 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{31}{2} = 15\) cm

Q42. A spherical mirror has radius of curvature 56 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{56}{2} = 28\) cm

Q43. A spherical mirror has radius of curvature 12 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{12}{2} = 6\) cm

Q44. A spherical mirror has radius of curvature 40 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{40}{2} = 20\) cm

Q45. A spherical mirror has radius of curvature 47 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{47}{2} = 23\) cm

Q46. A spherical mirror has radius of curvature 30 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{30}{2} = 15\) cm

Q47. A spherical mirror has radius of curvature 27 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{27}{2} = 13\) cm

Q48. A spherical mirror has radius of curvature 16 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{16}{2} = 8\) cm

Q49. A spherical mirror has radius of curvature 51 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{51}{2} = 25\) cm

Q50. A spherical mirror has radius of curvature 25 cm. Find its focal length.

Difficulty: Easy · Topic: Focal length from radius of curvature

Solution

\(f = \frac{R}{2} = \frac{25}{2} = 12\) cm

Q51. A concave mirror is also known as a:

Difficulty: Easy-Medium · Topic: Spherical Mirrors

Solution

A concave mirror is called a converging mirror because when parallel rays of light fall on it, they converge (come together) at the focus after reflection. A convex mirror is the diverging mirror — it spreads rays apart.

Q52. The radius of curvature of a spherical mirror is 30 cm. What is its focal length?

Difficulty: Easy-Medium · Topic: Spherical Mirrors

Solution

For a spherical mirror, the relationship between focal length (f) and radius of curvature (R) is:

f = R / 2

f = 30 / 2 = 15 cm

Q53. Which type of mirror is used as a rear-view mirror in vehicles and why?

Difficulty: Easy-Medium · Topic: Convex Mirror

Solution

Convex mirrors are used as rear-view mirrors because they always form erect and diminished images. More importantly, they have a wider field of view compared to plane mirrors, allowing the driver to see a larger area of the road behind. The images are always virtual, erect, and smaller than the object.

Q54. When a ray of light passes from air into glass, it:

Difficulty: Easy-Medium · Topic: Refraction

Solution

Glass is optically denser than air. When light enters a denser medium from a rarer medium, it bends toward the normal. Since the refractive index of glass (~1.5) is greater than air (~1.0), light slows down in glass. The speed of light in glass is approximately c/1.5 = 2 × 108 m/s.

Q55. Find the power of a concave lens of focal length 50 cm.

Difficulty: Easy-Medium · Topic: Power of a Lens

Solution

Given: f = -50 cm = -0.5 m (negative because concave lens)

Power: P = 1/f = 1/(-0.5) = -2 D

The negative sign indicates it is a diverging (concave) lens.

Q56. Differentiate between regular and diffuse reflection. Does diffuse reflection mean the failure of the laws of reflection?

Difficulty: Easy-Medium · Topic: Reflection

Solution

Regular Reflection: Occurs on smooth, polished surfaces. All reflected rays are parallel to each other, forming a clear image. Example: plane mirror, still water surface.

Diffuse Reflection: Occurs on rough, unpolished surfaces. Reflected rays scatter in different directions because the surface has micro-irregularities. Example: paper, wall, fabric.

Does diffuse reflection violate the laws of reflection?

No. Even in diffuse reflection, each individual ray obeys both laws of reflection perfectly. At each tiny point on the rough surface, the angle of incidence equals the angle of reflection. The scattering occurs because the normals at different points on the surface point in different directions due to the surface roughness.

Q57. A pencil partly immersed in water appears to be bent at the water surface. Explain this observation with the help of refraction.

Difficulty: Easy-Medium · Topic: Refraction

Solution

When we look at a pencil partly immersed in water, the part below the water surface appears displaced or bent. Here's why:

Light rays from the submerged part of the pencil travel from water (denser medium) to air (rarer medium). At the water-air interface, these rays bend away from the normal (refraction from denser to rarer medium).

When these refracted rays reach our eyes, our brain traces them back in straight lines. The apparent position of the submerged part of the pencil is where these extended straight lines seem to originate — which is higher than its actual position.

Since the part above water is at its true position and the part below appears shifted upward, the pencil appears to be bent or broken at the surface. This is an optical illusion caused by refraction.

Q58. When an object is placed at the focus of a convex lens, the image is formed:

Difficulty: Easy-Medium · Topic: Convex Lens

Solution

When an object is placed at the focus (F₁) of a convex lens, the refracted rays emerge parallel to the principal axis. Parallel rays never meet (or meet at infinity). Therefore, the image is formed at infinity. It is real, inverted, and highly enlarged. This is the principle behind searchlights and collimators.

Q59. A concave lens always produces an image that is:

Difficulty: Easy-Medium · Topic: Concave Lens

Solution

A concave lens is a diverging lens. It always diverges the rays passing through it. The diverging rays, when traced back, appear to meet on the same side as the object. This produces an image that is always virtual, erect, and diminished — regardless of where the object is placed.

Q60. The power of a lens is +2 D. Its focal length and type are:

Difficulty: Easy-Medium · Topic: Power of a Lens

Solution

P = 1/f (where f is in metres)

f = 1/P = 1/2 = 0.5 m = 50 cm

Since P is positive, it is a convex (converging) lens.

Q61. An object placed at -50 cm from a mirror produces an image at -40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-40}{-50} = -0.8\)

Q62. An object placed at -30 cm from a mirror produces an image at -15 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-15}{-30} = -0.5\)

Q63. An object placed at -35 cm from a mirror produces an image at -25 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-25}{-35} = -0.71\)

Q64. An object placed at -24 cm from a mirror produces an image at 40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{40}{-24} = 1.67\)

Q65. An object placed at -40 cm from a mirror produces an image at -20 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-20}{-40} = -0.5\)

Q66. An object placed at -45 cm from a mirror produces an image at 20 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{20}{-45} = 0.44\)

Q67. An object placed at -40 cm from a mirror produces an image at -30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-30}{-40} = -0.75\)

Q68. An object placed at -24 cm from a mirror produces an image at 20 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{20}{-24} = 0.83\)

Q69. An object placed at -25 cm from a mirror produces an image at 40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{40}{-25} = 1.6\)

Q70. An object placed at -30 cm from a mirror produces an image at 40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{40}{-30} = 1.33\)

Q71. An object placed at -50 cm from a mirror produces an image at -15 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-15}{-50} = -0.3\)

Q72. An object placed at -40 cm from a mirror produces an image at -15 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-15}{-40} = -0.38\)

Q73. An object placed at -60 cm from a mirror produces an image at -30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-30}{-60} = -0.5\)

Q74. An object placed at -50 cm from a mirror produces an image at -60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-60}{-50} = -1.2\)

Q75. An object placed at -45 cm from a mirror produces an image at -30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-30}{-45} = -0.67\)

Q76. An object placed at -24 cm from a mirror produces an image at -40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-40}{-24} = -1.67\)

Q77. An object placed at -25 cm from a mirror produces an image at -40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-40}{-25} = -1.6\)

Q78. An object placed at -24 cm from a mirror produces an image at -30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-30}{-24} = -1.25\)

Q79. An object placed at -40 cm from a mirror produces an image at -25 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-25}{-40} = -0.62\)

Q80. An object placed at -50 cm from a mirror produces an image at 30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{30}{-50} = 0.6\)

Q81. An object placed at -50 cm from a mirror produces an image at 20 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{20}{-50} = 0.4\)

Q82. An object placed at -60 cm from a mirror produces an image at -20 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-20}{-60} = -0.33\)

Q83. An object placed at -30 cm from a mirror produces an image at -60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-60}{-30} = -2.0\)

Q84. An object placed at -60 cm from a mirror produces an image at 40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{40}{-60} = 0.67\)

Q85. An object placed at -24 cm from a mirror produces an image at 60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{60}{-24} = 2.5\)

Q86. An object placed at -50 cm from a mirror produces an image at -30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-30}{-50} = -0.6\)

Q87. An object placed at -24 cm from a mirror produces an image at -20 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-20}{-24} = -0.83\)

Q88. An object placed at -60 cm from a mirror produces an image at 30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{30}{-60} = 0.5\)

Q89. An object placed at -24 cm from a mirror produces an image at -60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-60}{-24} = -2.5\)

Q90. An object placed at -20 cm from a mirror produces an image at -25 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-25}{-20} = -1.25\)

Q91. An object placed at -50 cm from a mirror produces an image at 60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{60}{-50} = 1.2\)

Q92. An object placed at -40 cm from a mirror produces an image at -40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-40}{-40} = -1.0\)

Q93. An object placed at -45 cm from a mirror produces an image at -25 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-25}{-45} = -0.56\)

Q94. An object placed at -15 cm from a mirror produces an image at -20 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-20}{-15} = -1.33\)

Q95. An object placed at -15 cm from a mirror produces an image at -30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-30}{-15} = -2.0\)

Q96. An object placed at -30 cm from a mirror produces an image at 60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{60}{-30} = 2.0\)

Q97. An object placed at -45 cm from a mirror produces an image at -40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-40}{-45} = -0.89\)

Q98. An object placed at -24 cm from a mirror produces an image at 30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{30}{-24} = 1.25\)

Q99. An object placed at -35 cm from a mirror produces an image at -30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-30}{-35} = -0.86\)

Q100. An object placed at -24 cm from a mirror produces an image at -25 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-25}{-24} = -1.04\)

Q101. An object placed at -45 cm from a mirror produces an image at 40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{40}{-45} = 0.89\)

Q102. An object placed at -60 cm from a mirror produces an image at 20 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{20}{-60} = 0.33\)

Q103. An object placed at -25 cm from a mirror produces an image at -60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-60}{-25} = -2.4\)

Q104. An object placed at -60 cm from a mirror produces an image at -25 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-25}{-60} = -0.42\)

Q105. An object placed at -35 cm from a mirror produces an image at -20 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-20}{-35} = -0.57\)

Q106. An object placed at -35 cm from a mirror produces an image at -60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-60}{-35} = -1.71\)

Q107. An object placed at -20 cm from a mirror produces an image at -30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-30}{-20} = -1.5\)

Q108. An object placed at -60 cm from a mirror produces an image at -40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-40}{-60} = -0.67\)

Q109. An object placed at -20 cm from a mirror produces an image at 30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{30}{-20} = 1.5\)

Q110. An object placed at -30 cm from a mirror produces an image at -25 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-25}{-30} = -0.83\)

Q111. An object placed at -20 cm from a mirror produces an image at 60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{60}{-20} = 3.0\)

Q112. An object placed at -45 cm from a mirror produces an image at -60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-60}{-45} = -1.33\)

Q113. An object placed at -60 cm from a mirror produces an image at 60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{60}{-60} = 1.0\)

Q114. An object placed at -15 cm from a mirror produces an image at 40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{40}{-15} = 2.67\)

Q115. An object placed at -35 cm from a mirror produces an image at 30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{30}{-35} = 0.86\)

Q116. An object placed at -50 cm from a mirror produces an image at -20 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-20}{-50} = -0.4\)

Q117. An object placed at -15 cm from a mirror produces an image at 60 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{60}{-15} = 4.0\)

Q118. An object placed at -30 cm from a mirror produces an image at -30 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-30}{-30} = -1.0\)

Q119. An object placed at -30 cm from a mirror produces an image at -40 cm. Find the magnification.

Difficulty: Easy-Medium · Topic: Magnification

Solution

\(m = -\frac{v}{u} = -\frac{-40}{-30} = -1.33\)

Q120. An object is placed between the focus F and the pole P of a concave mirror. The image formed is:

Difficulty: Medium · Topic: Concave Mirror

Solution

When an object is placed between F and P of a concave mirror, the reflected rays diverge. If we extend them behind the mirror, they appear to meet, forming a virtual, erect, and enlarged image. This is the principle used in shaving mirrors and makeup mirrors.

Q121. An object is placed at a distance of 30 cm from a concave mirror of focal length 15 cm. Find the position, nature, and magnification of the image.

Difficulty: Medium · Topic: Mirror Formula

Solution

Given: u = -30 cm (object is always on the left), f = -15 cm (concave mirror has negative f)

Using mirror formula:

1/v + 1/u = 1/f

1/v + 1/(-30) = 1/(-15)

1/v = -1/15 + 1/30

1/v = (-2 + 1)/30 = -1/30

v = -30 cm

The negative sign means the image is formed in front of the mirror — it is real.

Magnification:

m = -v/u = -(-30)/(-30) = -1

|m| = 1, so the image is the same size as the object. The negative sign indicates the image is inverted.

Nature: Real, inverted, same size, formed at the centre of curvature (since v = -30 cm = 2f = R).

Q122. An object is placed at a distance of 20 cm from a convex mirror of focal length 10 cm. Find the position and magnification of the image.

Difficulty: Medium · Topic: Convex Mirror

Solution

Given: u = -20 cm, f = +10 cm (convex mirror has positive f)

Using mirror formula:

1/v + 1/u = 1/f

1/v + 1/(-20) = 1/10

1/v = 1/10 + 1/20 = (2 + 1)/20 = 3/20

v = +20/3 ≈ +6.67 cm

Positive v means the image is behind the mirror — it is virtual.

Magnification:

m = -v/u = -(20/3)/(-20) = +1/3 ≈ +0.33

Positive m means the image is erect. |m| < 1 means it is diminished (one-third the size).

Q123. The refractive index of glass is 1.5. If the speed of light in vacuum is 3 × 10⁸ m/s, calculate the speed of light in glass.

Difficulty: Medium · Topic: Refractive Index

Solution

Given: n = 1.5, c = 3 × 108 m/s

Formula: n = c/v

Therefore: v = c/n = (3 × 108) / 1.5

v = 2 × 108 m/s

Light travels at two-thirds of its vacuum speed when passing through glass.

Q124. The refractive index of diamond is 2.42. This means:

Difficulty: Medium · Topic: Snell's Law

Solution

The refractive index n = c/v. A refractive index of 2.42 means:

2.42 = (speed in vacuum)/(speed in diamond)

So light travels 2.42 times slower in diamond compared to vacuum. This is why diamond bends light so dramatically and sparkles brilliantly — its high refractive index causes significant bending and internal reflections.

Q125. An object is placed 30 cm from a convex lens of focal length 20 cm. Find the position and nature of the image.

Difficulty: Medium · Topic: Lens Formula

Solution

Given: u = -30 cm (object on left), f = +20 cm (convex lens)

Using lens formula:

1/v - 1/u = 1/f

1/v - 1/(-30) = 1/20

1/v + 1/30 = 1/20

1/v = 1/20 - 1/30 = (3 - 2)/60 = 1/60

v = +60 cm

Positive v means the image is on the right side of the lens — it is real.

Magnification: m = v/u = 60/(-30) = -2

Negative m means the image is inverted. |m| = 2 means it is twice the size of the object (enlarged).

Q126. A doctor prescribes a corrective lens of power +1.5 D. Find the focal length of the lens. Is it a converging or diverging lens?

Difficulty: Medium · Topic: Power of a Lens

Solution

Given: P = +1.5 D

Focal length: f = 1/P = 1/1.5 = +0.667 m ≈ +66.7 cm

Since the power is positive, this is a converging (convex) lens.

A positive power lens is prescribed for hypermetropia (long-sightedness), where the person has difficulty seeing nearby objects.

Q127. Two thin lenses of power +3.5 D and -2.5 D are placed in contact. Find the power and focal length of the combination.

Difficulty: Medium · Topic: Combination of Lenses

Solution

Given: P₁ = +3.5 D, P₂ = -2.5 D

Net power: P = P₁ + P₂ = 3.5 + (-2.5) = +1 D

Focal length: f = 1/P = 1/1 = +1 m = 100 cm

The combination behaves like a converging lens (positive power) of focal length 100 cm.

Q128. Draw a ray diagram to show the image formation when an object is placed between the centre of curvature C and the focus F of a concave mirror. State the characteristics of the image.

Difficulty: Medium · Topic: Concave Mirror

Solution

Ray Diagram Description:

Place the object (arrow) between C and F on the principal axis of a concave mirror.

  • Ray 1: Draw a ray parallel to the principal axis from the top of the object to the mirror. After reflection, this ray passes through F.
  • Ray 2: Draw a ray from the top of the object passing through F to the mirror. After reflection, this ray becomes parallel to the principal axis.

The two reflected rays meet at a point beyond C, on the same side as the object. Draw the image (inverted arrow) at this point.

Characteristics of the image:

  • Position: Beyond the centre of curvature C
  • Size: Enlarged (larger than the object)
  • Nature: Real (formed by actual intersection of reflected rays)
  • Orientation: Inverted (upside down)
Q129. Why is a concave mirror used in headlights of cars? At what position should the bulb be placed?

Difficulty: Medium · Topic: Spherical Mirrors

Solution

A concave mirror is used in headlights because of a special property: when a light source is placed at the focus (F) of a concave mirror, the reflected rays emerge as a powerful parallel beam of light.

This is the reverse of what happens when parallel rays hit a concave mirror (they converge at F). By placing the bulb at F, we get the reverse — a strong, directed beam that illuminates the road ahead over long distances without scattering.

The bulb should be placed at the focus of the concave mirror. If the bulb is placed slightly above or below the focus, the beam spreads slightly, which is how low-beam/high-beam adjustments work in some headlight designs.

Q130. State Snell's law of refraction. The refractive index of water is 1.33 and that of glass is 1.52. Calculate the refractive index of glass with respect to water.

Difficulty: Medium · Topic: Refraction

Solution

Snell's Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media and for a given colour of light.

n₁ sin i = n₂ sin r

Or: sin i / sin r = n₂/n₁

Calculation:

Refractive index of glass with respect to water:

n(glass/water) = n(glass)/n(water) = 1.52/1.33 = 1.14

This means light travels 1.14 times slower in glass than in water. When light passes from water to glass, it bends toward the normal since glass is optically denser than water.

Q131. An object is placed at -25 cm from a concave mirror of focal length -10 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-10} - \frac{1}{-25}\)

\(v = -16.67\) cm

Q132. An object is placed at -50 cm from a concave mirror of focal length -30 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-30} - \frac{1}{-50}\)

\(v = -75.0\) cm

Q133. An object is placed at -45 cm from a concave mirror of focal length -14 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-14} - \frac{1}{-45}\)

\(v = -20.32\) cm

Q134. An object is placed at -30 cm from a concave mirror of focal length -30 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-30} - \frac{1}{-30}\)

\(v = infinity\) cm

Q135. An object is placed at -25 cm from a concave mirror of focal length -12 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-12} - \frac{1}{-25}\)

\(v = -23.08\) cm

Q136. An object is placed at -40 cm from a concave mirror of focal length -12 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-12} - \frac{1}{-40}\)

\(v = -17.14\) cm

Q137. An object is placed at -35 cm from a concave mirror of focal length -12 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-12} - \frac{1}{-35}\)

\(v = -18.26\) cm

Q138. An object is placed at -35 cm from a concave mirror of focal length -10 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-10} - \frac{1}{-35}\)

\(v = -14.0\) cm

Q139. An object is placed at -60 cm from a concave mirror of focal length -10 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-10} - \frac{1}{-60}\)

\(v = -12.0\) cm

Q140. An object is placed at -15 cm from a concave mirror of focal length -16 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-16} - \frac{1}{-15}\)

\(v = 240.0\) cm

Q141. An object is placed at -25 cm from a concave mirror of focal length -15 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-15} - \frac{1}{-25}\)

\(v = -37.5\) cm

Q142. An object is placed at -50 cm from a concave mirror of focal length -20 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-20} - \frac{1}{-50}\)

\(v = -33.33\) cm

Q143. An object is placed at -50 cm from a concave mirror of focal length -25 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-25} - \frac{1}{-50}\)

\(v = -50.0\) cm

Q144. An object is placed at -35 cm from a concave mirror of focal length -14 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-14} - \frac{1}{-35}\)

\(v = -23.33\) cm

Q145. An object is placed at -40 cm from a concave mirror of focal length -15 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-15} - \frac{1}{-40}\)

\(v = -24.0\) cm

Q146. An object is placed at -20 cm from a concave mirror of focal length -14 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-14} - \frac{1}{-20}\)

\(v = -46.67\) cm

Q147. An object is placed at -50 cm from a concave mirror of focal length -10 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-10} - \frac{1}{-50}\)

\(v = -12.5\) cm

Q148. An object is placed at -40 cm from a concave mirror of focal length -16 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-16} - \frac{1}{-40}\)

\(v = -26.67\) cm

Q149. An object is placed at -24 cm from a concave mirror of focal length -25 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-25} - \frac{1}{-24}\)

\(v = 600.0\) cm

Q150. An object is placed at -15 cm from a concave mirror of focal length -20 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-20} - \frac{1}{-15}\)

\(v = 60.0\) cm

Q151. An object is placed at -60 cm from a concave mirror of focal length -30 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-30} - \frac{1}{-60}\)

\(v = -60.0\) cm

Q152. An object is placed at -25 cm from a concave mirror of focal length -14 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-14} - \frac{1}{-25}\)

\(v = -31.82\) cm

Q153. An object is placed at -30 cm from a concave mirror of focal length -20 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-20} - \frac{1}{-30}\)

\(v = -60.0\) cm

Q154. An object is placed at -20 cm from a concave mirror of focal length -25 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-25} - \frac{1}{-20}\)

\(v = 100.0\) cm

Q155. An object is placed at -60 cm from a concave mirror of focal length -14 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-14} - \frac{1}{-60}\)

\(v = -18.26\) cm

Q156. Light travels from air into glass (refractive index = 1.52). If the angle of incidence is 55 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(55^\circ) = 1.52 \times \sin r\)

\(\sin r = \frac{\sin 55^\circ}{1.52}\)

\(r = 32.6^\circ\)

Q157. Light travels from air into glass (refractive index = 1.44). If the angle of incidence is 40 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(40^\circ) = 1.44 \times \sin r\)

\(\sin r = \frac{\sin 40^\circ}{1.44}\)

\(r = 26.5^\circ\)

Q158. Light travels from air into glass (refractive index = 1.33). If the angle of incidence is 50 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(50^\circ) = 1.33 \times \sin r\)

\(\sin r = \frac{\sin 50^\circ}{1.33}\)

\(r = 35.2^\circ\)

Q159. Light travels from air into glass (refractive index = 1.5). If the angle of incidence is 45 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(45^\circ) = 1.5 \times \sin r\)

\(\sin r = \frac{\sin 45^\circ}{1.5}\)

\(r = 28.1^\circ\)

Q160. Light travels from air into glass (refractive index = 1.55). If the angle of incidence is 55 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(55^\circ) = 1.55 \times \sin r\)

\(\sin r = \frac{\sin 55^\circ}{1.55}\)

\(r = 31.9^\circ\)

Q161. Light travels from air into glass (refractive index = 1.6). If the angle of incidence is 30 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(30^\circ) = 1.6 \times \sin r\)

\(\sin r = \frac{\sin 30^\circ}{1.6}\)

\(r = 18.2^\circ\)

Q162. Light travels from air into glass (refractive index = 1.55). If the angle of incidence is 25 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(25^\circ) = 1.55 \times \sin r\)

\(\sin r = \frac{\sin 25^\circ}{1.55}\)

\(r = 15.8^\circ\)

Q163. Light travels from air into glass (refractive index = 1.45). If the angle of incidence is 60 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(60^\circ) = 1.45 \times \sin r\)

\(\sin r = \frac{\sin 60^\circ}{1.45}\)

\(r = 36.7^\circ\)

Q164. Light travels from air into glass (refractive index = 1.55). If the angle of incidence is 60 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(60^\circ) = 1.55 \times \sin r\)

\(\sin r = \frac{\sin 60^\circ}{1.55}\)

\(r = 34.0^\circ\)

Q165. Light travels from air into glass (refractive index = 1.33). If the angle of incidence is 55 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(55^\circ) = 1.33 \times \sin r\)

\(\sin r = \frac{\sin 55^\circ}{1.33}\)

\(r = 38.0^\circ\)

Q166. Light travels from air into glass (refractive index = 1.55). If the angle of incidence is 30 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(30^\circ) = 1.55 \times \sin r\)

\(\sin r = \frac{\sin 30^\circ}{1.55}\)

\(r = 18.8^\circ\)

Q167. Light travels from air into glass (refractive index = 1.33). If the angle of incidence is 60 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(60^\circ) = 1.33 \times \sin r\)

\(\sin r = \frac{\sin 60^\circ}{1.33}\)

\(r = 40.6^\circ\)

Q168. Light travels from air into glass (refractive index = 1.7). If the angle of incidence is 30 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(30^\circ) = 1.7 \times \sin r\)

\(\sin r = \frac{\sin 30^\circ}{1.7}\)

\(r = 17.1^\circ\)

Q169. Light travels from air into glass (refractive index = 1.45). If the angle of incidence is 50 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(50^\circ) = 1.45 \times \sin r\)

\(\sin r = \frac{\sin 50^\circ}{1.45}\)

\(r = 31.9^\circ\)

Q170. Light travels from air into glass (refractive index = 1.52). If the angle of incidence is 25 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(25^\circ) = 1.52 \times \sin r\)

\(\sin r = \frac{\sin 25^\circ}{1.52}\)

\(r = 16.1^\circ\)

Q171. Light travels from air into glass (refractive index = 1.5). If the angle of incidence is 25 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(25^\circ) = 1.5 \times \sin r\)

\(\sin r = \frac{\sin 25^\circ}{1.5}\)

\(r = 16.4^\circ\)

Q172. Light travels from air into glass (refractive index = 1.33). If the angle of incidence is 25 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(25^\circ) = 1.33 \times \sin r\)

\(\sin r = \frac{\sin 25^\circ}{1.33}\)

\(r = 18.5^\circ\)

Q173. Light travels from air into glass (refractive index = 1.5). If the angle of incidence is 50 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(50^\circ) = 1.5 \times \sin r\)

\(\sin r = \frac{\sin 50^\circ}{1.5}\)

\(r = 30.7^\circ\)

Q174. Light travels from air into glass (refractive index = 1.5). If the angle of incidence is 30 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(30^\circ) = 1.5 \times \sin r\)

\(\sin r = \frac{\sin 30^\circ}{1.5}\)

\(r = 19.5^\circ\)

Q175. Light travels from air into glass (refractive index = 1.45). If the angle of incidence is 25 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(25^\circ) = 1.45 \times \sin r\)

\(\sin r = \frac{\sin 25^\circ}{1.45}\)

\(r = 16.9^\circ\)

Q176. Light travels from air into glass (refractive index = 1.45). If the angle of incidence is 40 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(40^\circ) = 1.45 \times \sin r\)

\(\sin r = \frac{\sin 40^\circ}{1.45}\)

\(r = 26.3^\circ\)

Q177. Light travels from air into glass (refractive index = 1.55). If the angle of incidence is 40 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(40^\circ) = 1.55 \times \sin r\)

\(\sin r = \frac{\sin 40^\circ}{1.55}\)

\(r = 24.5^\circ\)

Q178. Light travels from air into glass (refractive index = 1.7). If the angle of incidence is 60 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(60^\circ) = 1.7 \times \sin r\)

\(\sin r = \frac{\sin 60^\circ}{1.7}\)

\(r = 30.6^\circ\)

Q179. An object is placed 45 cm from a convex lens of focal length 20 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -45 cm (sign convention), f = 20 cm

\(\frac{1}{v} = \frac{1}{20} + \frac{1}{45}\)

v = 13.85 cm

Q180. An object is placed 25 cm from a convex lens of focal length 25 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -25 cm (sign convention), f = 25 cm

\(\frac{1}{v} = \frac{1}{25} + \frac{1}{25}\)

v = 12.5 cm

Q181. An object is placed 60 cm from a convex lens of focal length 25 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -60 cm (sign convention), f = 25 cm

\(\frac{1}{v} = \frac{1}{25} + \frac{1}{60}\)

v = 17.65 cm

Q182. An object is placed 25 cm from a convex lens of focal length 20 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -25 cm (sign convention), f = 20 cm

\(\frac{1}{v} = \frac{1}{20} + \frac{1}{25}\)

v = 11.11 cm

Q183. An object is placed 40 cm from a convex lens of focal length 10 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -40 cm (sign convention), f = 10 cm

\(\frac{1}{v} = \frac{1}{10} + \frac{1}{40}\)

v = 8.0 cm

Q184. An object is placed 40 cm from a convex lens of focal length 20 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -40 cm (sign convention), f = 20 cm

\(\frac{1}{v} = \frac{1}{20} + \frac{1}{40}\)

v = 13.33 cm

Q185. An object is placed 35 cm from a convex lens of focal length 20 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -35 cm (sign convention), f = 20 cm

\(\frac{1}{v} = \frac{1}{20} + \frac{1}{35}\)

v = 12.73 cm

Q186. An object is placed 35 cm from a convex lens of focal length 25 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -35 cm (sign convention), f = 25 cm

\(\frac{1}{v} = \frac{1}{25} + \frac{1}{35}\)

v = 14.58 cm

Q187. An object is placed 15 cm from a convex lens of focal length 20 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -15 cm (sign convention), f = 20 cm

\(\frac{1}{v} = \frac{1}{20} + \frac{1}{15}\)

v = 8.57 cm

Q188. An object is placed 35 cm from a convex lens of focal length 12 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -35 cm (sign convention), f = 12 cm

\(\frac{1}{v} = \frac{1}{12} + \frac{1}{35}\)

v = 8.94 cm

Q189. An object is placed 15 cm from a convex lens of focal length 10 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -15 cm (sign convention), f = 10 cm

\(\frac{1}{v} = \frac{1}{10} + \frac{1}{15}\)

v = 6.0 cm

Q190. An object is placed 15 cm from a convex lens of focal length 12 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -15 cm (sign convention), f = 12 cm

\(\frac{1}{v} = \frac{1}{12} + \frac{1}{15}\)

v = 6.67 cm

Q191. An object is placed 45 cm from a convex lens of focal length 25 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -45 cm (sign convention), f = 25 cm

\(\frac{1}{v} = \frac{1}{25} + \frac{1}{45}\)

v = 16.07 cm

Q192. An object is placed 35 cm from a convex lens of focal length 10 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -35 cm (sign convention), f = 10 cm

\(\frac{1}{v} = \frac{1}{10} + \frac{1}{35}\)

v = 7.78 cm

Q193. An object is placed 50 cm from a convex lens of focal length 25 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -50 cm (sign convention), f = 25 cm

\(\frac{1}{v} = \frac{1}{25} + \frac{1}{50}\)

v = 16.67 cm

Q194. An object is placed 30 cm from a convex lens of focal length 15 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -30 cm (sign convention), f = 15 cm

\(\frac{1}{v} = \frac{1}{15} + \frac{1}{30}\)

v = 10.0 cm

Q195. An object is placed 45 cm from a convex lens of focal length 12 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -45 cm (sign convention), f = 12 cm

\(\frac{1}{v} = \frac{1}{12} + \frac{1}{45}\)

v = 9.47 cm

Q196. An object is placed 20 cm from a convex lens of focal length 12 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -20 cm (sign convention), f = 12 cm

\(\frac{1}{v} = \frac{1}{12} + \frac{1}{20}\)

v = 7.5 cm

Q197. An object is placed 35 cm from a convex lens of focal length 15 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -35 cm (sign convention), f = 15 cm

\(\frac{1}{v} = \frac{1}{15} + \frac{1}{35}\)

v = 10.5 cm

Q198. An object is placed 50 cm from a convex lens of focal length 10 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -50 cm (sign convention), f = 10 cm

\(\frac{1}{v} = \frac{1}{10} + \frac{1}{50}\)

v = 8.33 cm

Q199. Two thin lenses of focal lengths 30 cm and -15 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{30} + \frac{1}{-15}\)

\(f = -30.0\) cm

Q200. Two thin lenses of focal lengths 10 cm and 15 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{10} + \frac{1}{15}\)

\(f = 6.0\) cm

Q201. Two thin lenses of focal lengths 15 cm and 25 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{15} + \frac{1}{25}\)

\(f = 9.38\) cm

Q202. Two thin lenses of focal lengths 15 cm and 30 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{15} + \frac{1}{30}\)

\(f = 10.0\) cm

Q203. Two thin lenses of focal lengths 20 cm and 20 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{20} + \frac{1}{20}\)

\(f = 10.0\) cm

Q204. Two thin lenses of focal lengths 10 cm and -20 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{10} + \frac{1}{-20}\)

\(f = 20.0\) cm

Q205. Two thin lenses of focal lengths 15 cm and -50 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{15} + \frac{1}{-50}\)

\(f = 21.43\) cm

Q206. Two thin lenses of focal lengths 30 cm and 30 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{30} + \frac{1}{30}\)

\(f = 15.0\) cm

Q207. Two thin lenses of focal lengths 20 cm and 15 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{20} + \frac{1}{15}\)

\(f = 8.57\) cm

Q208. Two thin lenses of focal lengths 20 cm and -15 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{20} + \frac{1}{-15}\)

\(f = -60.0\) cm

Q209. Two thin lenses of focal lengths 25 cm and -15 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{25} + \frac{1}{-15}\)

\(f = -37.5\) cm

Q210. Two thin lenses of focal lengths 25 cm and -20 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{25} + \frac{1}{-20}\)

\(f = -100.0\) cm

Q211. Two thin lenses of focal lengths 10 cm and -50 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{10} + \frac{1}{-50}\)

\(f = 12.5\) cm

Q212. Two thin lenses of focal lengths 15 cm and 10 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{15} + \frac{1}{10}\)

\(f = 6.0\) cm

Q213. Two thin lenses of focal lengths 20 cm and -30 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{20} + \frac{1}{-30}\)

\(f = 60.0\) cm

Q214. Two thin lenses of focal lengths 20 cm and -40 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{20} + \frac{1}{-40}\)

\(f = 40.0\) cm

Q215. Two thin lenses of focal lengths 20 cm and 30 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{20} + \frac{1}{30}\)

\(f = 12.0\) cm

Q216. Two thin lenses of focal lengths 10 cm and 25 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{10} + \frac{1}{25}\)

\(f = 7.14\) cm

Q217. Two thin lenses of focal lengths 25 cm and 30 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{25} + \frac{1}{30}\)

\(f = 13.64\) cm

Q218. Two thin lenses of focal lengths 10 cm and 30 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{10} + \frac{1}{30}\)

\(f = 7.5\) cm

Q219. Two thin lenses of focal lengths 15 cm and -40 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{15} + \frac{1}{-40}\)

\(f = 24.0\) cm

Q220. Two thin lenses of focal lengths 30 cm and 20 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{30} + \frac{1}{20}\)

\(f = 12.0\) cm

Q221. Two thin lenses of focal lengths 25 cm and 20 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{25} + \frac{1}{20}\)

\(f = 11.11\) cm

Q222. An object is placed at -40 cm from a concave mirror of focal length -8 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-8} - \frac{1}{-40}\)

\(v = -10.0\) cm

Q223. An object is placed at -20 cm from a concave mirror of focal length -10 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-10} - \frac{1}{-20}\)

\(v = -20.0\) cm

Q224. An object is placed at -60 cm from a concave mirror of focal length -15 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-15} - \frac{1}{-60}\)

\(v = -20.0\) cm

Q225. An object is placed at -45 cm from a concave mirror of focal length -18 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-18} - \frac{1}{-45}\)

\(v = -30.0\) cm

Q226. An object is placed at -25 cm from a concave mirror of focal length -20 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-20} - \frac{1}{-25}\)

\(v = -100.0\) cm

Q227. An object is placed at -60 cm from a concave mirror of focal length -18 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-18} - \frac{1}{-60}\)

\(v = -25.71\) cm

Q228. An object is placed at -30 cm from a concave mirror of focal length -18 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-18} - \frac{1}{-30}\)

\(v = -45.0\) cm

Q229. An object is placed at -45 cm from a concave mirror of focal length -15 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-15} - \frac{1}{-45}\)

\(v = -22.5\) cm

Q230. An object is placed at -60 cm from a concave mirror of focal length -25 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-25} - \frac{1}{-60}\)

\(v = -42.86\) cm

Q231. An object is placed at -30 cm from a concave mirror of focal length -15 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-15} - \frac{1}{-30}\)

\(v = -30.0\) cm

Q232. An object is placed at -30 cm from a concave mirror of focal length -10 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-10} - \frac{1}{-30}\)

\(v = -15.0\) cm

Q233. An object is placed at -25 cm from a concave mirror of focal length -16 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-16} - \frac{1}{-25}\)

\(v = -44.44\) cm

Q234. An object is placed at -35 cm from a concave mirror of focal length -18 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-18} - \frac{1}{-35}\)

\(v = -37.06\) cm

Q235. An object is placed at -35 cm from a concave mirror of focal length -30 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-30} - \frac{1}{-35}\)

\(v = -210.0\) cm

Q236. An object is placed at -30 cm from a concave mirror of focal length -14 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-14} - \frac{1}{-30}\)

\(v = -26.25\) cm

Q237. An object is placed at -60 cm from a concave mirror of focal length -8 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-8} - \frac{1}{-60}\)

\(v = -9.23\) cm

Q238. An object is placed at -50 cm from a concave mirror of focal length -18 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-18} - \frac{1}{-50}\)

\(v = -28.13\) cm

Q239. An object is placed at -35 cm from a concave mirror of focal length -20 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-20} - \frac{1}{-35}\)

\(v = -46.67\) cm

Q240. An object is placed at -35 cm from a concave mirror of focal length -15 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-15} - \frac{1}{-35}\)

\(v = -26.25\) cm

Q241. An object is placed at -40 cm from a concave mirror of focal length -18 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-18} - \frac{1}{-40}\)

\(v = -32.73\) cm

Q242. An object is placed at -45 cm from a concave mirror of focal length -16 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-16} - \frac{1}{-45}\)

\(v = -24.83\) cm

Q243. An object is placed at -50 cm from a concave mirror of focal length -15 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-15} - \frac{1}{-50}\)

\(v = -21.43\) cm

Q244. An object is placed at -15 cm from a concave mirror of focal length -10 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-10} - \frac{1}{-15}\)

\(v = -30.0\) cm

Q245. An object is placed at -24 cm from a concave mirror of focal length -18 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-18} - \frac{1}{-24}\)

\(v = -72.0\) cm

Q246. An object is placed at -60 cm from a concave mirror of focal length -20 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-20} - \frac{1}{-60}\)

\(v = -30.0\) cm

Q247. An object is placed at -15 cm from a concave mirror of focal length -12 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-12} - \frac{1}{-15}\)

\(v = -60.0\) cm

Q248. An object is placed at -35 cm from a concave mirror of focal length -16 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-16} - \frac{1}{-35}\)

\(v = -29.47\) cm

Q249. An object is placed at -35 cm from a concave mirror of focal length -8 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-8} - \frac{1}{-35}\)

\(v = -10.37\) cm

Q250. An object is placed at -40 cm from a concave mirror of focal length -14 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-14} - \frac{1}{-40}\)

\(v = -21.54\) cm

Q251. An object is placed at -30 cm from a concave mirror of focal length -16 cm. Find the image distance.

Difficulty: Medium · Topic: Mirror Formula

Solution

Mirror formula: \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\)

\(\frac{1}{v} = \frac{1}{-16} - \frac{1}{-30}\)

\(v = -34.29\) cm

Q252. Light travels from air into glass (refractive index = 1.6). If the angle of incidence is 35 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(35^\circ) = 1.6 \times \sin r\)

\(\sin r = \frac{\sin 35^\circ}{1.6}\)

\(r = 21.0^\circ\)

Q253. Light travels from air into glass (refractive index = 1.52). If the angle of incidence is 50 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(50^\circ) = 1.52 \times \sin r\)

\(\sin r = \frac{\sin 50^\circ}{1.52}\)

\(r = 30.3^\circ\)

Q254. Light travels from air into glass (refractive index = 1.7). If the angle of incidence is 55 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(55^\circ) = 1.7 \times \sin r\)

\(\sin r = \frac{\sin 55^\circ}{1.7}\)

\(r = 28.8^\circ\)

Q255. Light travels from air into glass (refractive index = 1.45). If the angle of incidence is 35 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(35^\circ) = 1.45 \times \sin r\)

\(\sin r = \frac{\sin 35^\circ}{1.45}\)

\(r = 23.3^\circ\)

Q256. Light travels from air into glass (refractive index = 1.5). If the angle of incidence is 55 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(55^\circ) = 1.5 \times \sin r\)

\(\sin r = \frac{\sin 55^\circ}{1.5}\)

\(r = 33.1^\circ\)

Q257. Light travels from air into glass (refractive index = 1.45). If the angle of incidence is 55 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(55^\circ) = 1.45 \times \sin r\)

\(\sin r = \frac{\sin 55^\circ}{1.45}\)

\(r = 34.4^\circ\)

Q258. Light travels from air into glass (refractive index = 1.44). If the angle of incidence is 45 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(45^\circ) = 1.44 \times \sin r\)

\(\sin r = \frac{\sin 45^\circ}{1.44}\)

\(r = 29.4^\circ\)

Q259. Light travels from air into glass (refractive index = 1.7). If the angle of incidence is 25 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(25^\circ) = 1.7 \times \sin r\)

\(\sin r = \frac{\sin 25^\circ}{1.7}\)

\(r = 14.4^\circ\)

Q260. Light travels from air into glass (refractive index = 1.7). If the angle of incidence is 35 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(35^\circ) = 1.7 \times \sin r\)

\(\sin r = \frac{\sin 35^\circ}{1.7}\)

\(r = 19.7^\circ\)

Q261. Light travels from air into glass (refractive index = 1.6). If the angle of incidence is 45 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(45^\circ) = 1.6 \times \sin r\)

\(\sin r = \frac{\sin 45^\circ}{1.6}\)

\(r = 26.2^\circ\)

Q262. Light travels from air into glass (refractive index = 1.33). If the angle of incidence is 40 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(40^\circ) = 1.33 \times \sin r\)

\(\sin r = \frac{\sin 40^\circ}{1.33}\)

\(r = 28.9^\circ\)

Q263. Light travels from air into glass (refractive index = 1.52). If the angle of incidence is 35 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(35^\circ) = 1.52 \times \sin r\)

\(\sin r = \frac{\sin 35^\circ}{1.52}\)

\(r = 22.2^\circ\)

Q264. Light travels from air into glass (refractive index = 1.7). If the angle of incidence is 40 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(40^\circ) = 1.7 \times \sin r\)

\(\sin r = \frac{\sin 40^\circ}{1.7}\)

\(r = 22.2^\circ\)

Q265. Light travels from air into glass (refractive index = 1.45). If the angle of incidence is 45 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(45^\circ) = 1.45 \times \sin r\)

\(\sin r = \frac{\sin 45^\circ}{1.45}\)

\(r = 29.2^\circ\)

Q266. Light travels from air into glass (refractive index = 1.44). If the angle of incidence is 60 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(60^\circ) = 1.44 \times \sin r\)

\(\sin r = \frac{\sin 60^\circ}{1.44}\)

\(r = 37.0^\circ\)

Q267. Light travels from air into glass (refractive index = 1.52). If the angle of incidence is 40 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(40^\circ) = 1.52 \times \sin r\)

\(\sin r = \frac{\sin 40^\circ}{1.52}\)

\(r = 25.0^\circ\)

Q268. Light travels from air into glass (refractive index = 1.55). If the angle of incidence is 35 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(35^\circ) = 1.55 \times \sin r\)

\(\sin r = \frac{\sin 35^\circ}{1.55}\)

\(r = 21.7^\circ\)

Q269. Light travels from air into glass (refractive index = 1.55). If the angle of incidence is 45 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(45^\circ) = 1.55 \times \sin r\)

\(\sin r = \frac{\sin 45^\circ}{1.55}\)

\(r = 27.1^\circ\)

Q270. Light travels from air into glass (refractive index = 1.6). If the angle of incidence is 50 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(50^\circ) = 1.6 \times \sin r\)

\(\sin r = \frac{\sin 50^\circ}{1.6}\)

\(r = 28.6^\circ\)

Q271. Light travels from air into glass (refractive index = 1.52). If the angle of incidence is 30 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(30^\circ) = 1.52 \times \sin r\)

\(\sin r = \frac{\sin 30^\circ}{1.52}\)

\(r = 19.2^\circ\)

Q272. Light travels from air into glass (refractive index = 1.5). If the angle of incidence is 40 degrees, find the angle of refraction (to 1 decimal place).

Difficulty: Medium · Topic: Snell's Law

Solution

Snell's Law: \(n_1 \sin i = n_2 \sin r\)

\(1 \times \sin(40^\circ) = 1.5 \times \sin r\)

\(\sin r = \frac{\sin 40^\circ}{1.5}\)

\(r = 25.4^\circ\)

Q273. An object is placed 50 cm from a convex lens of focal length 12 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -50 cm (sign convention), f = 12 cm

\(\frac{1}{v} = \frac{1}{12} + \frac{1}{50}\)

v = 9.68 cm

Q274. An object is placed 25 cm from a convex lens of focal length 10 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -25 cm (sign convention), f = 10 cm

\(\frac{1}{v} = \frac{1}{10} + \frac{1}{25}\)

v = 7.14 cm

Q275. An object is placed 60 cm from a convex lens of focal length 20 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -60 cm (sign convention), f = 20 cm

\(\frac{1}{v} = \frac{1}{20} + \frac{1}{60}\)

v = 15.0 cm

Q276. An object is placed 20 cm from a convex lens of focal length 20 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -20 cm (sign convention), f = 20 cm

\(\frac{1}{v} = \frac{1}{20} + \frac{1}{20}\)

v = 10.0 cm

Q277. An object is placed 40 cm from a convex lens of focal length 15 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -40 cm (sign convention), f = 15 cm

\(\frac{1}{v} = \frac{1}{15} + \frac{1}{40}\)

v = 10.91 cm

Q278. An object is placed 60 cm from a convex lens of focal length 10 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -60 cm (sign convention), f = 10 cm

\(\frac{1}{v} = \frac{1}{10} + \frac{1}{60}\)

v = 8.57 cm

Q279. An object is placed 60 cm from a convex lens of focal length 12 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -60 cm (sign convention), f = 12 cm

\(\frac{1}{v} = \frac{1}{12} + \frac{1}{60}\)

v = 10.0 cm

Q280. An object is placed 20 cm from a convex lens of focal length 25 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -20 cm (sign convention), f = 25 cm

\(\frac{1}{v} = \frac{1}{25} + \frac{1}{20}\)

v = 11.11 cm

Q281. An object is placed 30 cm from a convex lens of focal length 12 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -30 cm (sign convention), f = 12 cm

\(\frac{1}{v} = \frac{1}{12} + \frac{1}{30}\)

v = 8.57 cm

Q282. An object is placed 45 cm from a convex lens of focal length 10 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -45 cm (sign convention), f = 10 cm

\(\frac{1}{v} = \frac{1}{10} + \frac{1}{45}\)

v = 8.18 cm

Q283. An object is placed 25 cm from a convex lens of focal length 12 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -25 cm (sign convention), f = 12 cm

\(\frac{1}{v} = \frac{1}{12} + \frac{1}{25}\)

v = 8.11 cm

Q284. An object is placed 30 cm from a convex lens of focal length 10 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -30 cm (sign convention), f = 10 cm

\(\frac{1}{v} = \frac{1}{10} + \frac{1}{30}\)

v = 7.5 cm

Q285. An object is placed 50 cm from a convex lens of focal length 15 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -50 cm (sign convention), f = 15 cm

\(\frac{1}{v} = \frac{1}{15} + \frac{1}{50}\)

v = 11.54 cm

Q286. An object is placed 15 cm from a convex lens of focal length 25 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -15 cm (sign convention), f = 25 cm

\(\frac{1}{v} = \frac{1}{25} + \frac{1}{15}\)

v = 9.38 cm

Q287. An object is placed 20 cm from a convex lens of focal length 15 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -20 cm (sign convention), f = 15 cm

\(\frac{1}{v} = \frac{1}{15} + \frac{1}{20}\)

v = 8.57 cm

Q288. An object is placed 20 cm from a convex lens of focal length 10 cm. Find the image distance.

Difficulty: Medium · Topic: Lens formula - image distance

Solution

Lens formula: \(\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\)

u = -20 cm (sign convention), f = 10 cm

\(\frac{1}{v} = \frac{1}{10} + \frac{1}{20}\)

v = 6.67 cm

Q289. Two thin lenses of focal lengths 10 cm and -15 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{10} + \frac{1}{-15}\)

\(f = 30.0\) cm

Q290. Two thin lenses of focal lengths 30 cm and 25 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{30} + \frac{1}{25}\)

\(f = 13.64\) cm

Q291. Two thin lenses of focal lengths 15 cm and -20 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{15} + \frac{1}{-20}\)

\(f = 60.0\) cm

Q292. Two thin lenses of focal lengths 20 cm and -50 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{20} + \frac{1}{-50}\)

\(f = 33.33\) cm

Q293. Two thin lenses of focal lengths 15 cm and -30 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{15} + \frac{1}{-30}\)

\(f = 30.0\) cm

Q294. Two thin lenses of focal lengths 25 cm and 25 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{25} + \frac{1}{25}\)

\(f = 12.5\) cm

Q295. Two thin lenses of focal lengths 10 cm and -30 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{10} + \frac{1}{-30}\)

\(f = 15.0\) cm

Q296. Two thin lenses of focal lengths 25 cm and -40 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{25} + \frac{1}{-40}\)

\(f = 66.67\) cm

Q297. Two thin lenses of focal lengths 30 cm and -20 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{30} + \frac{1}{-20}\)

\(f = -60.0\) cm

Q298. Two thin lenses of focal lengths 25 cm and 10 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{25} + \frac{1}{10}\)

\(f = 7.14\) cm

Q299. Two thin lenses of focal lengths 30 cm and 15 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{30} + \frac{1}{15}\)

\(f = 10.0\) cm

Q300. Two thin lenses of focal lengths 30 cm and -30 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{30} + \frac{1}{-30}\)

\(f = infinity\) cm

Q301. Two thin lenses of focal lengths 20 cm and 25 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{20} + \frac{1}{25}\)

\(f = 11.11\) cm

Q302. Two thin lenses of focal lengths 10 cm and 20 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{10} + \frac{1}{20}\)

\(f = 6.67\) cm

Q303. Two thin lenses of focal lengths 30 cm and -40 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{30} + \frac{1}{-40}\)

\(f = 120.0\) cm

Q304. Two thin lenses of focal lengths 10 cm and 10 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{10} + \frac{1}{10}\)

\(f = 5.0\) cm

Q305. Two thin lenses of focal lengths 20 cm and -20 cm are placed in contact. Find the equivalent focal length.

Difficulty: Medium · Topic: Combination of thin lenses

Solution

\(\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}\)

\(= \frac{1}{20} + \frac{1}{-20}\)

\(f = infinity\) cm

Q306. A 5 cm tall object is placed at a distance of 20 cm from a concave mirror of focal length 15 cm. Find the position, size, and nature of the image.

Difficulty: Medium-Hard · Topic: Mirror Formula

Solution

Given: h = 5 cm, u = -20 cm, f = -15 cm

Step 1: Find v using mirror formula

1/v + 1/u = 1/f

1/v + 1/(-20) = 1/(-15)

1/v = -1/15 + 1/20

1/v = (-4 + 3)/60 = -1/60

v = -60 cm

Negative v means the image is formed 60 cm in front of the mirror — it is real.

Step 2: Find magnification and image height

m = -v/u = -(-60)/(-20) = -3

h' = m × h = -3 × 5 = -15 cm

The negative sign of h' means the image is inverted (below the principal axis).

Answer: The image is formed 60 cm in front of the mirror, is 15 cm tall, real, inverted, and 3 times enlarged.

Q307. A 4 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 24 cm. The distance of the object from the lens is 16 cm. Find the position, size, and nature of the image.

Difficulty: Medium-Hard · Topic: Lens Formula

Solution

Given: h = 4 cm, u = -16 cm, f = +24 cm

Note: The object is placed between F and O (u < f), so we expect a virtual image.

Step 1: Lens formula

1/v - 1/u = 1/f

1/v - 1/(-16) = 1/24

1/v + 1/16 = 1/24

1/v = 1/24 - 1/16 = (2 - 3)/48 = -1/48

v = -48 cm

Negative v means the image is on the same side as the object — it is virtual.

Step 2: Magnification and image size

m = v/u = (-48)/(-16) = +3

h' = m × h = 3 × 4 = +12 cm

Positive m and positive h' mean the image is erect. The image is 12 cm tall — 3 times enlarged.

Nature: Virtual, erect, enlarged, on the same side as the object.

Q308. A concave mirror produces a 3 times magnified real image of an object placed 10 cm in front of it. Find the focal length of the mirror and the position of the image.

Difficulty: Medium-Hard · Topic: Mirror Formula

Solution

Given: u = -10 cm, m = -3 (real image is inverted, so m is negative for 3× magnification)

Step 1: Find v using magnification

m = -v/u

-3 = -v/(-10)

-3 = v/10

v = -30 cm

Negative v confirms the image is real (in front of the mirror).

Step 2: Find f using mirror formula

1/v + 1/u = 1/f

1/(-30) + 1/(-10) = 1/f

-1/30 - 1/10 = 1/f

(-1 - 3)/30 = 1/f

-4/30 = 1/f

f = -30/4 = -7.5 cm

The focal length is 7.5 cm. The negative sign confirms it is a concave mirror.

Q309. An object 5 cm high is placed at a distance of 10 cm from a convex mirror of radius of curvature 30 cm. Find the nature, position, and size of the image.

Difficulty: Medium-Hard · Topic: Mirror Formula

Solution

Given: h = 5 cm, u = -10 cm, R = +30 cm (convex mirror)

f = R/2 = +30/2 = +15 cm

Step 1: Mirror formula

1/v + 1/u = 1/f

1/v + 1/(-10) = 1/15

1/v = 1/15 + 1/10 = (2 + 3)/30 = 5/30 = 1/6

v = +6 cm

Positive v means image is behind the mirror — virtual.

Step 2: Magnification and image height

m = -v/u = -(6)/(-10) = +0.6

h' = m × h = 0.6 × 5 = +3 cm

Positive m and h' confirm the image is erect. |m| < 1 so it is diminished.

Answer: Virtual, erect image formed 6 cm behind the mirror, 3 cm tall.

Q310. An object is placed at a distance of 15 cm from a concave lens of focal length 10 cm. Find the position and nature of the image. Also find the magnification.

Difficulty: Medium-Hard · Topic: Lens Formula

Solution

Given: u = -15 cm, f = -10 cm (concave lens has negative f)

Lens formula:

1/v - 1/u = 1/f

1/v - 1/(-15) = 1/(-10)

1/v + 1/15 = -1/10

1/v = -1/10 - 1/15 = (-3 - 2)/30 = -5/30 = -1/6

v = -6 cm

Negative v for a lens means the image is on the same side as the object — it is virtual.

Magnification:

m = v/u = (-6)/(-15) = +0.4

Positive m means the image is erect. |m| = 0.4 < 1, so the image is diminished (40% of original size).

Q311. A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in contact. Find the power and focal length of the combination. What is the nature of this combination?

Difficulty: Hard · Topic: Lens Formula

Solution

Given: f₁ = +25 cm = +0.25 m (convex), f₂ = -10 cm = -0.10 m (concave)

Power of individual lenses:

P₁ = 1/f₁ = 1/0.25 = +4 D

P₂ = 1/f₂ = 1/(-0.10) = -10 D

Net power:

P = P₁ + P₂ = 4 + (-10) = -6 D

Focal length of combination:

f = 1/P = 1/(-6) = -0.1667 m = -16.67 cm

Since the net power is negative, the combination acts as a diverging (concave) lens. The concave lens dominates because its power magnitude (10 D) exceeds that of the convex lens (4 D).

Other Chapters in Physics

Ch 2: The Human Eye and the Colourful WorldCh 3: ElectricityCh 4: Magnetic Effects of Electric CurrentCh 5: Sources of Energy

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